In the following description, in order to simplify the description, the designation "suction roll" will be used instead of suction member. This does not, however, restrict the invention in any way whatsoever, but rather the term suction roll refers to various suction rolls, suction cylinders, and equivalent suction members.
With respect to the prior art, reference is first made to the current assignee's Finnish Patent No. 83,680 (corresponding to U.S. Pat. No. 5,022,163, which is incorporated by reference herein), which describes a suction roll for supporting the paper web which does not comprise a suction box in the interior of the roll. Rather, the suction roll is constructed such that it comprises perforations in the roll mantle and, at the ends of the perforations, a separate recess, preferably a groove, through which the vacuum is distributed over a wider area on the roll face so as to produce a suitable suction force across the paper web. In this manner, with a favorable dimensioning of the perforations, an adequate holding force to hold the web is obtained without the need to place a suction box or an equivalent arrangement in the interior of the suction roll mantle.
When air is removed out of a revolving roll whose mantle is perforated and whose interior is empty, through a hollow centrally-located shaft or suction pipe in the shaft, in the suction pipe in the shaft a vortex is formed, which produces a high flow resistance and, thus, makes the removal of air from the roll mantle interior more difficult. With respect to the prior art related to this concept, reference is also made to the current assignee's Finnish Pat. No. 82,849 (corresponding to U.S. Pat. No. 5,024,729, which is incorporated by reference herein), which describes an embodiment related to the problems described above. In the suction roll in accordance with FI Pat. No. 82,849, it is considered a novelty that the suction roll comprises at least a vortex prevention equipment placed at the vicinity of the suction pipe of the suction roll. This vortex prevention equipment comprises at least one plate part which has a face substantially parallel to the radius of the roll, i.e., transverse to the axial direction of the roll, and which vortex prevention equipment is arranged in such a way in relation to the end of the suction pipe that, in an operative suction situation, increasing vortex formation in the sucked air is prevented and, thus, the vacuum level/negative pressure force is kept at a desired, substantially invariable value on the inside face of the roll mantle across the entire width of the roll. Also, the flow rate of suction air through the perforations in the roll mantle is kept at a desired, substantially invariable value. However, in this prior art construction, problems have still been caused by the pressure loss produced by the centrifugal force arising from the speed of rotation.